Paper is generally manufactured by dispersing cellulosic fiber in an aqueous medium and then removing most of the liquid. The paper derives some of its structural integrity from the mechanical interlocking of the cellulosic fibers in the web, but most by far of the paper's strength is derived from hydrogen bonding which links the cellulosic fibers to one another. With paper intended for use as bathroom tissue, the degree of strength imparted by this inter-fiber bonding, while necessary to the utility of the product, can result in a lack of perceived softness that is inimical to consumer acceptance. One common method of increasing the perceived softness and cushion of bathroom tissue is to crepe the paper. Creping is generally effected by fixing the cellulosic web to a Yankee drier with an adhesive/release agent combination and then scraping the web off the Yankee by means of a creping blade. Creping, by breaking a significant number of inter-fiber bonds, adds to and increases the perceived softness of resulting bathroom tissue product. However, creping with a conventional blade alone may not be sufficient to impart the desired combinations of softness, bulk and appearance.
We have discovered that tissue having highly desirable bulk, appearance and softness characteristics, can be produced by a process similar to conventional processes, particularly conventional wet pressing, except that the conventional creping blade is replaced with an undulatory creping blade presenting differentiated creping and rake angles to the sheet and having a multiplicity of spaced serrulated creping sections of either uniform depths or non-uniform arrays of depths. The depths of the undulations are above about 0.008 inches.
Techniques for creping of tissue and towel weight papers using patterned or non-uniform creping blades are known but these known techniques rather than being suitable for production of premium quality bath tissue, facial tissue or kitchen toweling, have been suggested for, and seem more suited for, production of wadding or insulating papers or other extremely coarse papers.
Three references of interest are Fuerst, U.S. Pat. No. 3,507,745; B. D. Nobbe, U.S. Pat. No. 3,163,575; and possibly British Patent 456,032. Fuerst, U.S. Pat. No. 3,507,745, suggests use of a highly beveled blade which has square shouldered notches formed into the rake surface. This type of a blade is said to be suitable for producing very high bulk for cushioning and insulation purposes but, in our opinion, is not suitable for premium quality towel and tissue products. The depth of the Fuerst blades' notches are only about 0.0015 inches to 0.007 inches.
Nobbe, U.S. Pat. No. 3,163,575, describes a doctor blade for differentially creping sheets from a drum to produce a product which is quite similar to that of the Fuerst patent. The Nobbe patent describes a blade with a relatively flat bevel angle into which notches have been cut, defining regions having a very large bevel angle. The crepe in the portions of the sheet that contact the notched portions of the blade will have quite a coarse crepe or no crepe, while the areas of the sheet that contact the unnotched blade portions will have a fine crepe.
In the Fuerst patent, the unmodified blade has a very large bevel angle, with portions of its creping edge being flattened to produce a surface that results in fine crepe in the portion of the sheet that contact this surface. The portions of the sheet that contact the unmodified sections of the blade will have very coarse crepe, thus giving an appearance of having almost no crepe. Our experience suggests that neither the Nobbe nor the Fuerst blades are suitable for the manufacture of commercially acceptable premium quality tissue and towel products.
Pashley, British Patent 456,032, teaches creping of a sheet from a drum using a creping blade whose edge has been serrated in a sawtooth pattern, the teeth being about one-eight (0.125) inch deep and numbering about 8 to the inch. The distance from tip to base of these teeth is about 2 to about 25 times the depth of the undulations that are cut into the present crepe blade. The product described in the Pashley patent has crepe that is much coarser and more irregular than the crepe of a product made using conventional creping technology. While this type of product may hold some advantages in the manufacture of crepe wadding, a product having such a coarse crepe would not normally be considered acceptable for use in premium tissue and towel products.
What has been needed is a simple, reliable process for creping tissue weight substrates to produce desirable products having higher caliper at lower basis weight than are produced in processes using a conventional creping blade. Products made using the creping procedure of the present invention will have a crepe fineness similar to that of conventionally-made tissue sheets but the resulting web combines crepe bars extending in the cross direction with undulations extending in the machine direction.